The present invention relates to a scanning electron microscope that scans the surface of a specimen with an electron beam and forms a two-dimensional electron image representing the shape or composition of the surface of the specimen through the detection of secondary signals produced by the specimen. More particularly, the present invention relates to a scanning electron beam microscope suitable for forming electron beam images of a high resolution at a high throughput by rapidly moving an observation point to tens of test positions on a semiconductor wafer as a specimen.
A scanning electron microscope (hereinafter abbreviated to xe2x80x9cSEMxe2x80x9d) accelerates electrons emitted by an electron source of a heating electron emission type or a field electron emission type, collimates the accelerated electrons in a fine electron beam, i.e., a primary electron beam using an electrostatic lens or a magnetic field lens, scans a specimen two-dimensionally with the primary electron beam, detects secondary electrons generated by the specimen irradiated with the primary electron beam or secondary signal electrons, i.e., reflected electrons, and forms a two-dimensional electron image by applying intensities of detection signals as brightness modulating inputs to a cathode-ray tube (abbreviated to xe2x80x9cCRTxe2x80x9d) that is scanned in synchronism with a scanning operation using the primary electron beam.
Device miniaturization has progressively advanced in the semiconductor industry in recent years, and optical microscopes for inspection in semiconductor device fabricating processes and test processes have been replaced by SEMs. The SEM uses an electron beam for dimension measurement and testing electrical operations. When observing an insulating specimen, such as a wafer that is used in the semiconductor industry, is observed with a SEM, a low acceleration voltage of 1 kV or below must be used not to charge the insulating specimen. Generally, the resolution of a general SEM using a low acceleration voltage of 1 kV is about 10 nm. As the miniaturization of semiconductor devices advances, demand for SEMs capable of forming images in a high resolution by using a low acceleration voltage has increased. A retarding system and a boosting system were developed and proposed in, for example, Japanese Patent Laid-open No. Hei 9-171791 to meet such demand. Those previously proposed systems enable observation in a resolution of about 3 nm under optimum conditions for observation.
When a SEM is used for the inspection of a semiconductor device during semiconductor device fabricating processes or a completed semiconductor device, capability of rapidly moving an observation point to tens of inspection positions on a semiconductor wafer is a prerequisite of the SEM for the improvement of the throughput of an inspection process. Therefore, a stage capable of rapid movement has been used. However, the positioning accuracy of the stage is on the order of several micrometers. Mechanical control of the position of the stage in an accuracy on the order of nanometers is economically infeasible and is practically difficult in respect of moving speed. Therefore, to position the stage in a high accuracy higher than several micrometers, there is adopted an image shifting system that shifts electrically the coordinates of the scanning center of a primary electron beam. In some cases, since the coordinates are shifted by a distance as long as several micrometers, the image shifting system employed in the conventional SEM deteriorates resolution when the distance of shift is great.
According to the present invention it is an object of the present invention to provide a SEM capable of image shifting an image without causing significant deterioration of resolution.
With the foregoing object in view, the present invention provides a SEM comprising: an electron source, an image shifting deflector system including two deflectors disposed respectively at upper and lower stages to shift an irradiation position of a primary electron beam emitted by the electron source on a specimen; and an objective that focuses the primary electron beam; wherein the objective has a lens gap opening toward the specimen, and the deflectors disposed at the lower stage on the side of the specimen forms a deflecting electric field in a region corresponding to an effective principal plane of the objective.